The present invention relates to a structure of a pressure deriving port of an internal combustion engine, for detecting pressure of a suction system thereof.
Examples of a conventional pressure deriving port are shown in Japanese Utility Model Laid-open Nos. 138037/1982, 35646/1984 and 178630/1986 and FIG. 3 shows a suction system equipped with a pressure deriving port according to either of them. In FIG. 3, a reference numeral 1 depicts an air cleaner and 2 a throttle body connected to the air cleaner 1, which is coposed of a fuel injector 3 for injecting fuel under control of a fuel control device which is not shown and a throttle valve 4 to be controlled by an operator, etc. An intake manifold 5 is disposed between the throttle body 2 and an engine 9 and includes an exhaust gas recirculating port 6, a blow-by gas discharge port 7 and a distributor 8 for distributing a mixture of air and fuel injected by the injector 3 to respective cylinders of the engine 9.
FIG. 4 is an upsteam side view of the intake manifold 5 in which an opening 10 of the manifold 5 to be connected to the throttle body 2 and a pressure deriving port 11. A filter constituted with fine metal wires or porous resin, etc., is fitted in the pressure deriving port 11 and a rubber tube is connected to the filter for guiding pressure to a pressure sensor which may be one having a gage resister formed by semiconductor diffusion and a diaphram portion.
Air taken in through the air cleaner 1 is mixed with fuel injected by the injector 3 and a resulatant air-fuel mixture is regulated in flow rate by a degree of opening of the throttle valve and distributed through the distributor 5 to the respective cylinders of the engine 9. For purification of exhaust gas, the latter and blo-by gas of amounts predetermined by operating conditions of the engine are discharged from the exhaust gas recirculating port 6 and the blow-by gas discharge port 7 to the intake manifold 5, respectively. The pressure sensor connected through the rubber tube to the pressure deriving port 11 detects a variation of pressure in the intake manifold 5 and provides it as a voltage signal which is supplied to a microcomputer of the fuel control device to control fuel injection.
In this construction, since there are exhaust gas, blow-by gas, water content of air and fuel exist in the intake manifold 5 which are pulsated due to suction stroke of the engine and reach a sensor element of the pressure sensor through the rubber tube connecting the pressure deriving port 11 to the pressure sensor, the water content may be frozen on a surface of the tube to clog the latter or on a surface of the diaphram of the pressure sensor element to cause a pressure indication of the pressure sensor to be erroneous. Further, carbon and/or oil components may deposited on the tube surface and/or the element surface to cause their characteristics to be changed.
In order to solve such problem as mentioned above, it is usual in the conventional device to provide such filter or to make the pressure passage complicated as shown in Japanese Utility Model Laid-open No. 138037/1982. However, these solutions require an increased number of manufacturing steps as well as parts, resulting in high cost.